This invention relates to a droplet generator for a continuous stream ink jet print head.
More particularly the invention relates to such a generator comprising: an elongate cavity for containing the ink; nozzle orifices in a wall of said cavity for passing ink from the cavity to form jets, said nozzle orifices extending along the length of said cavity; and actuator means disposed on the opposite side of said cavity to said wall for vibrating the ink in said cavity such that each said jet breaks up into ink droplets, in operation of said generator a standing wave being present in the ink in said cavity.
An example of a generator as described in the preceding paragraph is disclosed in U.S. Pat. No. 5,502,473. The generator of U.S. Pat. No. 5,502,473 is designed to operate at, or very close to, a frequency at which its ink cavity is resonant in the vertical direction, i.e. from the actuator means to the nozzle orifices. This requires a very high accuracy in the physical dimensions of the structural components of the generator. It also permits very little stray in operating parameters of the generator such as ink composition and temperature.
U.S. Pat. No. 4,827,287 discloses a droplet generator for a continuous stream ink jet print head. In this generator, in order to achieve the required jet break up, a travelling wave is caused to travel along an elongate nozzle orifice containing plate by means of the direct physical vibration of one end of the plate. The area of the plate free to vibrate is narrowed in the direction of wave propagation to compensate for attenuative losses. U.S. Pat. No. 4,827,287 uses what is called a waveguided construction.
According to the present invention there is provided a droplet generator for a continuous stream ink jet print head comprising: an elongate cavity for containing the ink; nozzle orifices in a wall of said cavity for passing ink from the cavity to form jets, said nozzle orifices extending along the length of said cavity; and actuator means disposed on the opposite side of said cavity to said wall for vibrating the ink in said cavity such that each said jet breaks up into ink droplets, in operation of said generator a standing wave being present in the ink in said cavity, characterised in that the cross-sectional area of said cavity varies along its length in a manner so as to tailor the form of said standing wave in the cavity such that each said jet breaks up into ink droplets at a respective predetermined distance from said wall of the cavity.
Preferably, the tailoring of the form of said standing wave is such that each said jet breaks up into ink droplets at substantially the same predetermined distance from said wall of the cavity.
Preferably, in the case of the preceding paragraph, the cross-sectional area of the cavity varies cyclically along its length between minimum and maximum values, said cross-sectional area having a minimum value whereat said standing wave has a region of low acoustic pressure at the nozzle orifices, said cross-sectional area having a maximum value whereat said standing wave has a region of high acoustic pressure at the nozzle orifices. The cyclical variation is for example sinusoidal.
In the case of each of the preceding two paragraphs, the cavity suitably has a generally triangular or a generally rectangular cross-section.
The invention also provides a method of operating a generator according to each of the preceding three paragraphs wherein the parameters of the operation of the generator are permitted to stray such that said cavity operates over a range extending substantially all the way between two successive resonances in the length of the cavity. Typically the parameters of the operation permitted to stray are ink composition and temperature.
The invention further provides a method of operating a generator according to each of the aforementioned three paragraphs wherein the cavity operates at substantially midway between two successive resonances in the length of the cavity.
A droplet generator in accordance with the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: